Reduction of oxides of nitrogen in a gas stream using boron-containing molecular sieve CHA

ABSTRACT

A process for reduction of oxides of nitrogen contained in a gas stream comprising contacting the gas stream with a boron-containing molecular sieve, the boron-containing molecular sieve having the CHA crystal structure and comprising (1) silicon oxide and (2) boron oxide or a combination of boron oxide and aluminum oxide, iron oxide, titanium oxide, gallium oxide and mixtures thereof; and wherein the mole-ratio of silicon oxide to boron oxide in said boron-containing molecular sieve is between 15 and 125. A method for reduction of oxides of nitrogen, comprising a) selecting the boron-containing molecular sieve; and b) contacting a gas stream with the molecular sieve. A method for reduction of oxides, comprising: a) selecting the boron-containing molecular sieve containing a metal or metal ions capable of catalyzing the reduction of the oxides of nitrogen; and b) contacting an exhaust stream of an internal combustion engine with the molecular sieve.

This application claims the benefit under 35 USC 119 of Provisional Application No. 60/632,005, filed Nov. 30, 2004 and is a Continuation-in-Part of Non Provisional application Ser. No. 11/266,115, filed Nov. 2, 2005 now abandoned.

BACKGROUND

Chabazite, which has the crystal structure designated “CHA”, is a natural zeolite with the approximate formula Ca₆Al₁₂Si₂₄O₇₂. Synthetic forms of chabazite are described in “Zeolite Molecular Sieves” by D. W. Breck, published in 1973 by John Wiley & Sons. The Synthetic forms reported by Breck are: zeolite “K-G”, described in J. Chem. Soc., p. 2822 (1956), Barrer et al.; zeolite D, described in British Patent No. 868,846 (1961); and zeolite R, described in U.S. Pat. No. 3,030,181, issued Apr. 17, 1962 to Milton et al. Chabazite is also discussed in “Atlas of Zeolite Structure Types” (1978) by W. H. Meier and D. H. Olson.

The K-G zeolite material reported in the J. Chem. Soc. Article by Barrer et al. is a potassium form having a silica:alumina mole ratio (referred to herein as “SAR”) of 2.3:1 to 4.15:1. Zeolite D reported in British Patent No. 868,846 is a sodium-potassium form having a SAR of 4.5:1 to 4.9:1. Zeolite R reported in U.S. Pat. No. 3,030,181 is a sodium form which has a SAR of 3.45:1 to 3.65:1.

Citation No. 9366052y in Volume 93 (1980) of Chemical Abstracts concerns a Russian language article by Tsitsishrili et al. in Soobsch. Akad. Nauk. Gruz. SSR 1980, 97(3) 621-4. This article teaches that the presence of tetramethylammonium ions in a reaction mixture containing K₂O—Na₂O—SiO₂—Al₂O₃—H₂O promotes the crystallization of chabazite. The zeolite obtained by the crystallization procedure has a SAR of 4.23.

The molecular sieve designated SSZ-13, which has the CHA crystal structure, is disclosed in U.S. Pat. No. 4,544,538, issued Oct. 1, 1985 to Zones. SSZ-13 is prepared from nitrogen-containing cations derived from 1-adamantamine, 3-quinuclidinol and 2-exo-aminonorbornane. Zones discloses that the SSZ-13 of U.S. Pat. No. 4,544,538 has a composition, as-synthesized and in the anhydrous state, in terms of mole ratios of oxides as follows:

(0.5 to 1.4)R₂O: (0 to 0.5)M₂O:W₂O₃: (greater than 5) YO₂ wherein M is an alkali metal cation, W is selected from aluminum, gallium and mixtures thereof, Y is selected from silicon, germanium and mixtures thereof, and R is an organic cation. U.S. Pat. No. 4,544,538 does not, however, disclose boron-containing SSZ-13.

U.S. Pat. No. 6,709,644, issued Mar. 23, 2004 to Zones et al., discloses zeolites having the CHA crystal structure and having small crystallite sizes. It does not, however, disclose a CHA zeolite containing boron. It is disclosed that the zeolite can be used for separation of gasses (e.g., separating carbon dioxide from natural gas), and in catalysts used for the reduction of oxides of nitrogen in a gas stream (e.g., automotive exhaust), converting lower alcohols and other oxygenated hydrocarbons to liquid products, and for producing dimethylamine.

U.S. Patent Publication US 2003/0176751A1 discloses zeolites having the CHA crystal structure with a silica/alumina molar ratio below and above 265. The reaction mixture with hydrofluoric acid used to produce the zeolite has a low Wt % yield of zeolite based on silica. It also does not produce zeolites having the CHA crystal structure wherein the mole ratio of silicon oxide to boron oxide in the zeolite is between 15 and 125.

SUMMARY OF THE INVENTION

There is provided a process for the reduction of oxides of nitrogen contained in a gas stream wherein said process comprises contacting the gas stream with a boron-containing molecular sieve, the boron-containing molecular sieve having the CHA crystal structure and comprising (1) silicon oxide, and (2) boron oxide or a combination of boron oxide and aluminum oxide, iron oxide, titanium oxide, gallium oxide and mixtures thereof; and wherein the mole ratio of silicon oxide to boron oxide in said boron-containing molecular sieve is between 15 and 125. The molecular sieve may contain oxide (2) wherein more than 50% of Oxide (2) is boron oxide on a molar basis. The molecular sieve may contain a metal or metal ions (such as cobalt, copper, platinum, iron, chromium, manganese, nickel, zinc, lanthanum, palladium, rhodium or mixtures thereof capable of catalyzing the reduction of the oxides of nitrogen, and the process may be conducted in the presence of a stoichiometric excess of oxygen. In one embodiment, the gas stream is the exhaust-stream of an internal combustion engine.

There, is, also provided a method for reduction of oxides of nitrogen contained in a gas stream, comprising:

a. selecting a boron-containing molecular sieve having a CHA-crystal structure and a mole ratio of silicon oxide to boron oxide between 15 and 125; and

b. contacting the gas stream with the boron-containing molecular sieve.

In another embodiment, there is provided a method for reduction of oxides of nitrogen contained in a gas stream, comprising:

a. selecting a boron-containing molecular sieve having a CHA crystal structure and a mole ratio of silicon oxide to boron oxide between 15 and 125; and containing a metal or metal ions capable of catalyzing the reduction of the oxides of nitrogen; and

b. contacting an exhaust stream of an internal combustion engine with the boron-containing molecular sieve.

DETAILED DESCRIPTION

The present invention relates to molecular sieves having the CHA crystal structure and containing boron in their crystal framework.

Boron-containing, CHA molecular sieves can be suitably prepared from an aqueous reaction mixture containing sources, of sources of an oxide of silicon; sources of boron oxide or a combination of boron oxide and aluminum oxide, iron oxide, titanium oxide, gallium oxide and mixtures thereof; optionally sources of an alkali metal or alkaline earth metal oxide; and a cation derived from 1-adamantamine, 3-quinuclidinol or 2-exo-aminonorbornane. The mixture should have a composition in terms of mole ratios falling within the ranges shown in Table A below:

TABLE A YO₂/W_(a)O_(b)   >2-2,000 OH—/YO₂ 0.2-0.45 Q/YO₂ 0.2-0.45 M_(2/n)O/YO₂   0-0.25 H₂O/YO₂ 22-80  wherein Y is silicon; W is boron or a combination of boron and aluminum, iron, titanium, gallium and mixtures thereof; M is an alkali metal or alkaline earth metal; n is the valence of M (i.e., 1 or 2) and Q is a quaternary ammonium cation derived from 1-adamantamine, 3-quinuclidinol or 2-exo-aminonorbornane (commonly known as a structure directing agent or “SDA”).

The quaternary ammonium cation derived from 1-adamantamine can be a N,N,N-trialkyl-1-adamantammonium cation which has the formula:

where R¹, R², and R³ are each independently a lower alkyl, for example methyl. The cation is associated with an anion, A⁻, which is not detrimental to the formation of the molecular sieve. Representative of such anions include halogens, such as fluoride, chloride, bromide and iodide; hydroxide; acetate; sulfate and carboxylate. Hydroxide is the preferred anion. It may be beneficial to ion exchange, for example, a halide for hydroxide ion, thereby reducing or eliminating the alkali metal or alkaline earth metal hydroxide required.

The quaternary ammonium cation derived from 3-quinuclidinol can have the formula:

where R₁ is defined as above.

The quaternary ammonium cation derived from 2-exo-aminonorbornane can have the formula:

where R¹, R², R³ and A are as defined above.

The reaction mixture is prepared using standard molecular sieve preparation techniques. Typical sources of silicon oxide include fumed silica, silicates, silica hydrogel, silicic acid, colloidal silica, tetra-alkyl orthosilicates, and silica hydroxides. Sources of boron oxide include borosilicate glasses and other reactive boron compounds. These include borates, boric acid and borate esters. Typical sources of aluminum oxide include aluminates, alumina, hydrated aluminum hydroxides, and aluminum compounds such as AlCl₃ and Al₂(SO₄)₃. Sources of other oxides are analogous to those for silicon oxide, boron oxide and aluminum oxide.

It has been found that seeding the reaction mixture with CHA crystals both directs and accelerates the crystallization, as well as minimizing the formation of undesired contaminants. In order to produce pure phase boron-containing CHA crystals, seeding may be required. When seeds are used, they can be used in an amount that is about 2-3 weight percent based on the weight of YO₂.

The reaction mixture is maintained at an elevated temperature until CHA crystals are formed. The temperatures during the hydrothermal crystallization step are typically maintained from about 120° C. to about 160° C. It has been found that a temperature below 160° C., e.g., about 120° C. to about 140° C., is useful for producing boron-containing CHA crystals without the formation of secondary crystal phases.

The crystallization period is typically greater than 1 day and preferably from about 3 days to about 7 days. The hydrothermal crystallization is conducted under pressure and usually in, an autoclave so that the reaction mixture is subject to autogenous pressure. The reaction mixture can be stirred, such as by rotating the reaction vessel, during crystallization.

Once the boron-containing CHA crystals have formed, the solid product is separated from the reaction mixture by standard mechanical separation techniques such as filtration. The crystals are water-washed and then dried, e.g. r, at 90° C. to 150° C. for from 8 to 24 hours, to obtain the as-synthesized crystals. The drying step can be performed at atmospheric or subatmospheric pressures.

The boron-containing CHA molecular sieve has a composition, as-synthesized and in the anhydrous state, in terms of mole ratios of oxides as indicated in Table B below:

As-Synthesized Boron-Containing CHA Composition

TABLE B YO₂/W_(c)O_(d) 20-2,000 M_(2/n)O/YO₂ 0-0.03 Q/YO₂ 0.02-0.05   where Y, W, M, n and Q are as defined above.

The boron-containing CHA molecular sieves, as-synthesized, have a crystalline structure whose X-ray powder diffraction (“XRD”) pattern shows the following characteristic lines:

TABLE I As-Synthesized Boron-Containing CHA XRD d-spacing Relative 2 Theta^((a)) (Angstroms) Intensity^((b)) 9.68 9.13 S 14.17 6.25 M 16.41 5.40 VS 17.94 4.94 M 21.13 4.20 VS 25.21 3.53 VS 26.61 3.35 W-M 31.11 2.87 M 31.42 2.84 M 31.59 2.83 M ^((a))±0.10 ^((b))The X-ray patterns provided are based on a relative intensity scale in which the strongest line in the X-ray pattern is assigned a value of 100: W(weak) is less than 20; M(medium) is between 20 and 40; S(strong) is between 40 and 60; VS(very strong) is greater than 60.

Table IA below shows the X-ray powder diffraction lines for as-synthesized boron-containing CHA including actual relative intensities.

TABLE IA As-Synthesized Boron-Containing CHA XRD d-spacing Relative 2 Theta^((a)) (Angstroms) Intensity (%) 9.68 9.13 55.2 13.21 6.70 5.4 14.17 6.25 33.5 16.41 5.40 81.3 17.94 4.94 32.6 19.43 4.56 6.8 21.13 4.20 100 22.35 3.97 15.8 23.00 3.86 10.1 23.57 3.77 5.1 25.21 3.53 78.4 26.61 3.35 20.2 28.37 3.14 6.0 28.57 3.12 4.4 30.27 2.95 3.9 31.11 2.87 29.8 31.42 2.84 38.3 31.59 2.83 26.5 32.27 2.77 1.4 33.15 2.70 3.0 33.93 2.64 4.7 35.44 2.53 3.9 35.84 2.50 1.2 36.55 2.46 10.9 39.40 2.29 1.8 40.02 2.25 1.3 40.44 2.23 1.0 40.73 2.21 6.0 ^((a))±0.10

After calcination, the boron-containing CHA molecular sieves have a crystalline structure whose X-ray powder diffraction pattern include the characteristic lines shown in Table II:

TABLE II Calcined Boron-Containing CHA XRD d-spacing Relative 2 Theta^((a)) (Angstroms) Intensity 9.74 9.07 VS 13.12 6.74 M 14.47 6.12 W 16.38 5.41 W 18.85 4.78 M 21.07 4.21 M 25.98 3.43 W 26.46 3.37 W 31.30 2.86 W 32.15 2.78 W ^((a))±0.10

Table IIA below shows the X-ray powder diffraction lines for calcined; boron-containing CHA including actual relative intensities.

TABLE IIA Calcined Boron-Containing CHA XRD d-spacing Relative 2 Theta^((a)) (Angstroms) Intensity (%) 9.74 9.07 100 13.12 6.74 29.5 14.47 6.12 4.6 16.38 5.41 14.2 18.85 4.78 22.1 19.60 4.53 2.2 21.07 4.21 32.9 22.84 3.89 2.2 23.68 3.75 0.8 25.98 3.43 13.1 26.46 3.37 8.7 28.27 3.15 1.3 9.74 9.07 100 13.12 6.74 29.5 14.47 6.12 4.6 16.38 5.41 14.2 18.85 4.78 22.1 19.60 4.53 2.2 21.07 4.21 32.9 29.24 3.05 1.6 30.32 2.95 1.7 31.30 2.86 14.4 32.15 2.78 9.0 32.56 2.75 0.2 35.26 2.54 2.4 ^((a))±0.10

The X-ray powder diffraction patterns were determined by standard techniques. The radiation was the K-alpha/doublet of copper and a scintillation counter spectrometer with a strip-chart pen recorder was used. The peak heights I and the positions, as a function of 2 Theta where Theta is the Bragg angle, were read from the spectrometer chart. From these measured values, the relative intensities, 100×I/Io, where Io is the intensity of the strongest line or peak, and d, the interplanar spacing in Angstroms corresponding to the recorded lines, can be calculated.

Variations in the diffraction pattern can result from variations in the mole ratio of oxides from sample to sample. The molecular sieve produced by exchanging the metal or other cations present in the molecular sieve with various other cations yields a similar diffraction pattern, although there can be shifts in interplanar spacing as well as variations in relative intensity. Calcination can also cause shifts in the X-ray diffraction pattern. Also, the symmetry can change based on the relative amounts of boron and aluminum in the crystal structure. Notwithstanding these perturbations, the basic crystal lattice structure remains unchanged.

Boron containing CHA molecular sieves may be used for the catalytic reduction of the oxides of nitrogen in a gas stream. Typically, the gas stream also contains oxygen, often a stoichiometric excess thereof. Also, the molecular sieve may contain a metal, or metal ions, within or on it which are capable of catalyzing the reduction of the nitrogen oxides. Examples of such metals or metal ions include cobalt, copper, platinum, iron, chromium, manganese nickel, zinc, lanthanum, palladium, rhodium and mixtures thereof.

One example of such a process for the catalytic reduction of oxides of nitrogen in the presence of a zeolite is disclosed in U.S. Pat. No. 4,297,328, issued Oct. 27, 1981 to Ritscher et al., which is incorporated by reference herein. There, the catalytic process is the combustion of carbon monoxide and hydrocarbons and the catalytic reduction of the oxides of nitrogen contained in, a gas stream, such as the exhaust gas from an internal combustion engine. The zeolite used is metal ion-exchanged, doped or loaded sufficiently so as to provide an effective amount of catalytic copper metal or copper ions within or on the zeolite. In addition, the process is conducted in an excess of oxidant, e.g., oxygen.

EXAMPLES Examples 1-14

Boron-containing CHA was synthesized by preparing the gel compositions i.e., reaction mixtures, having the compositions, in terms of mole ratios, shown in the table below. The resulting gel was placed in a Parr bomb reactor and heated in an oven at the temperature indicated below while rotating at the speed indicated below. Products were analyzed by X-ray diffraction (XRD) and found to be, boron-containing molecular sieves having the CHA structure. The source of silicon oxide was Cabosil M-5 fumed silica or HiSil 233 amorphous silica (0.208 wt. % alumina). The source of boron oxide was boric acid and the source of aluminum oxide was Reheis F 2000 alumina.

Ex. # SiO₂/B₂O₃ SiO₂/Al₂O₃ H₂O/SiO₂ OH—/SiO₂ Na+/SiO₂ SDA/SiO₂ Rx Cond.¹ Seeds %1-ada² 1 2.51 1,010 23.51 0.25 0.20 0.25 140/43/5d yes 100 2 12.01 1,010 22.74 0.25 0.08 0.25 140/43/5d yes 100 3 12.33 1,010 22.51 0.25 0.08 0.25 140/43/5d yes 100 4 12.07 288,900 23.00 0.26 0.09 0.26 140/43/5d no 100 5 12.33 37,129 22.51 0.25 0.09 0.25 140/43/5d yes 100 6 12.33 248,388 22.51 0.25 0.09 0.25 140/43/5d yes 100 7 12.33 248,388 22.53 0.25 0.09 0.25 140/43/5d yes 100 8 12.33 248,388 22.53 0.25 0.00 0.25 140/43/5d yes 100 9 12.33 248,388 22.51 0.25 0.09 0.25 160/43/4d yes 100 10 11.99 288,900 23.18 0.26 0.09 0.26 160/43/4d no 100 11 12.13 288,900 32.22 0.43 0.21 0.21 160/43/4d no 100 12 11.99 288,900 23.16 0.26 0.00 0.26 160/43/4d no 100 13 11.99 288,900 23.18 0.26 0.09 0.26 160/43/4d no 100 14 3.08 248,388 22.51 0.25 0.00 0.25 160/43/6d yes 100 ¹° C./RPM/Days ²1-ada = Quaternary ammonium cation derived from 1-adamantamine

The mole ratios of silicon oxide to boron oxide, and the Wt % yield on a silica basis, measured, in some of the Examples of boron-containing CHA molecular sieves are shown below.

Example Wt % Yield Based on # SiO₂/B₂O₃ Silica 6 39 90.5 8 43 91.0 11 45 61.8 13 39 85.5

Examples 15-20 Deboronation

Boron was removed from samples of the molecular sieves prepared as described in Example 13 above and then calcined. The sample was heated in an acid solution under the conditions indicated in the table below. The results are shown in the table.

Example. # Untreated Deboronation Rx (B) SSZ-13 15 16 17 18 19 20 Acid used — Acetic acid acetic acid acetic acid HCl HCl HCl Acid — 1.0 M 0.01 M 0.0001 M 0.01 M 0.001 M 0.0001 M Molarity Rx Cond. — 45 C./0 rpm/19 hr 45 C./0 rpm/19 hr 45 C./0 rpm/19 hr 45 C./0 rpm/19 hr 45 C./0 rpm/19 hr 45 C./0 rpm/19 hr Analysis Results Untreated Treated Treated Treated Treated Treated Treated Boron 0.66% 614 ppm 513 ppm 420 ppm 421 ppm 506 ppm 552 ppm

Comparative Examples 21-23

Aluminum and boron-containing CHA were synthesized according to the process of Example 1 in U.S. Patent Publication US2003/0176751. Comparative Example 21 used the same reaction mixture as in the patent publication, which was a mixture of ROH(R═N,N,N-trimethyladamantammoinium) solution, Al(NO₃)₃9H₂O and tetraethylorthosilicate. Comparative Example 22 replaced an equimolar amount of the aluminum nitrate with boric acid. Comparative Example 23 replaced a double molar amount of the aluminum nitrate with boric acid.

The reactions were conducted in a plastic beaker until the weights of the formed gels were reduced. The gels were ground to a powder with mortar and pestle and placed into a Teflon lined autoclave. Then 1.6 g of 49% aqueous hydrofluoric acid was stirred in.

The mixtures were crystallized in an autoclave heated to 150° C. and tumbled at 43 rpm for 65 hours. X-ray diffraction analyses of the samples at 65 hours showed that all three comparative examples were clean highly crystalline chabazites. After cooling, the mixtures were washed to low conductivity (<80 micromho/cm) to remove extraneous aluminum or boron. Analysis of the products, and calculation of Wt % yield on a silica basis, gave the results as shown below:

Exam- Si, Al, B, Wt % ple No. wt % wt % wt % SiO2/Al2O3 SiO2/B2O3 Yield 21 35.8 0.54 <25 128 >11057 20.5 ppm 22 35.8 0.0103 0.0768 6703 360 20.7 23 33.8 0.0098 0.1740 6652 150 7.3

None of these comparative examples had a mole ratio of silicon oxide to boron oxide of between 15 and 125; even when a doubling of the molar concentration of boric acid was added to the reaction mixture. The reaction mixture with hydrofluoric acid used to produce the zeolite has a low Wt % yield of zeolite based on silica. The Wt %-yields on a silica basis were all below 50 wt %. 

1. A process for reduction of oxides of nitrogen contained in a gas stream wherein said process comprises contacting the gas stream with a boron-containing molecular sieve, the boron-containing molecular sieve having the CHA crystal structure and comprising an oxide (1) comprising silicon oxide and an oxide (2) comprising boron oxide or a combination of boron oxide and aluminum oxide, iron oxide, titanium oxide, gallium oxide and mixtures thereof; and wherein the mole ratio of silicon oxide to boron oxide in said boron-containing molecular sieve is between 15 and
 50. 2. The process of claim 1 wherein the oxide (2) is more than 50% boron oxide on a molar basis.
 3. The process of claim 1 conducted in the presence of oxygen.
 4. The process of claim 1 wherein said boron-containing molecular sieve contains a metal or metal ions capable of catalyzing the reduction of the oxides of nitrogen.
 5. The process of claim 4 wherein the metal is cobalt, copper, platinum, iron, chromium, manganese, nickel, zinc, lanthanum, palladium, rhodium or mixtures thereof.
 6. The process of claim 1 wherein the gas stream is an exhaust stream of an internal combustion engine.
 7. The process of claim 5 wherein the gas stream is an exhaust stream of an internal combustion engine.
 8. A method for reduction of oxides of nitrogen contained in a gas stream, comprising: a. selecting a boron-containing molecular sieve having a CHA crystal structure and a mole ratio of silicon oxide to boron oxide between 15 and 50; and b. contacting the gas stream with the boron-containing molecular sieve.
 9. The method of claim 8, wherein the boron-containing molecular sieve is produced in a process with a Wt % yield based on silica of greater than
 50. 10. The method of claim 9, wherein the Wt % yield based on silica is greater than
 60. 11. The method of claim 10, wherein the Wt % yield based on silica is greater than
 80. 12. The method of claim 11, wherein the Wt % yield based on silica is greater than
 90. 13. A method for reduction of oxides of nitrogen contained in a gas stream, comprising: a. selecting a boron-containing molecular sieve having a CHA crystal structure and a mole ratio of silicon oxide to boron oxide between 15 and 50; and containing a metal or metal ions capable of catalyzing the reduction of the oxides of nitrogen; and b. contacting an exhaust stream of an internal combustion engine with the boron-containing molecular sieve.
 14. The method of claim 13, wherein the boron-containing molecular sieve is produced in a process with a Wt % yield based on silica of greater than
 50. 15. The method of claim 14, wherein the boron-containing molecular sieve is produced in a process with a Wt % yield based on silica of greater than
 60. 